NELSON COUNTY, Va. (WTVR) – For days we’ve been telling you how the FBI is involved with the search for missing teen Alexis Murphy. Wednesday night CBS 6 found out what their role is and why they’re now part of the case.

Alexis disappeared Saturday night, after telling her family she was going shopping in Lynchburg.

Her car was found 72 hours later in a Charlottesville movie theatre parking lot.

“I beg you from the bottom of my heart: help get her home,” said her aunt on Wednesday.

The teen’s family says her disappearance is out of character for Alexis, a rising senior who they say is very attached to her family.

Wednesday Nelson County detectives, along with FBI agents, knocked on doors at the Arden Place apartments in Charlottesville after a K-9 had tracked a scent to the complex that sits right behind the theatre parking lot where her car was found.

“You don’t assume this is a runaway,” said retired FBI agent James McNamara. “The FBI jumps in with both feet to try to find the child, and are not asking themselves if they should be involved or not.”

***SIDEBAR: Mother very familiar with experience of searching for missing child talks about key to finding Alexis Murphy***

McNamara says the bureau gets involved for several reasons when a child is younger than 18.

“The FBI offers extra investigative eyes, analytical support, evidence support and also the resources of the FBI laboratory in Quantico that’s close by,” he said.

More resources can mean faster results. That’s what the FBI shoots for in missing children cases. They also rely on the viewing public to play a part in getting young Alexis home.

“We are really feeling the pain,” her aunt said. “Please work with the authorities to bring her home safe and sound.”

Thursday will be day five since Alexis Murphy disappeared. There was a false sighting at a Charlottesville 7-11 Wednesday afternoon.

Retired agent McNamara stresses the importance of tips, no matter how small and insignificant you may think your information might be.

Word is spreading across the state about Alexis Murphy’s disappearance thanks in part to Old Dominion Truck Leasing, where the teen’s grandfather works.

Old Dominion Truck Leasing’s Buzz Ryder said the company emailed photos of Alexis to hundreds of drivers to make sure they keep an eye out for the 17-year-old.

“Just think, it could happen, if it happened to him, it could happen to one of our kids too,” one driver said.

And the word has spread to others drivers from other companies as well.

Stay with CBS 6 News and WTVR.com for the latest on this developing story.